1. Field of Invention
The present invention relates to a light emitting device driver circuit, a light emitting device array controller, and a light emitting device array control method; particularly, it relates to a light emitting device array controller and a control method with less number of chip pins, which do not reduce power utilization efficiency, and a light emitting device driver circuit using the light emitting device array controller.
2. Description of Related Art
One form of light emitting device which is commonly used nowadays is light emitting diode (LED). In large-size applications, LEDs are often arranged in an array, as a light source to provide backlight. As shown in FIG. 1, for controlling and supplying power to an LED array 40, an LED array controller 10 is required in an LED driver circuit, which controls a power supply stage 60 to provide a constant current to every LED string in the LED array 40. The LED array controller 10 is usually an integrated circuit (IC) chip.
More specifically, as shown in FIG. 1, the power supply stage 60 is controlled by the LED array controller 10 to convert an input voltage Vin to an output voltage Vout which is provided to the LED array 40. The LED array 90 includes multiple LED strings CH1-CHn, and every LED string has multiple LEDs connected in series. One end of each of the LED strings CH1-CHn is commonly coupled to the power supply stage 60, and the other end of each of the LED strings CH1-CHn is coupled to one end of a corresponding current source. Each current source controls the current through a corresponding LED string, such that the LEDs emit light uniformly.
In many nowadays applications, the LED array controller 10 is required to drive high power LEDs operating under power such as 1 W to 3 W and current such as 300 mA to 1 A. In such high power application, due to heat dissipation and cost issues of a large-sized chip, transistors of the current sources can not be integrated into the chip but have to be located externally, as shown in FIG. 1. In this case, if the transistors of the current sources are metal oxide semiconductor field effect transistors (MOSFET), the LED array controller 10 needs to provide three pins for each LED channel to electrically connect the source, gate, and drain of the external MOSFET respectively, wherein the pins for the source and the gate are required for the basic structure of the current source, and the drain signal must also be transmitted to the inside of the chip, for determining an appropriate output voltage Vout by feedback control.
FIG. 2 shows another prior art circuit, which is different from FIG. 1 in that the transistors of the current sources are PNP bipolar junction transistors (BJT) instead of MOSFETs. Because the location for obtaining the feedback signal is different, in this scheme, the LED array controller 10 only needs to provide two pins for each LED channel. The drawback of this scheme is that the voltage dropout of the current source is relatively large; the voltage dropout between the collector and the emitter of the PNP BJT is larger than 0.8 volts. The power utilization efficiency is relatively low.
In view of the foregoing, the present invention provides a light emitting device driver circuit, a light emitting device array controller and a control method thereof to reduce the number of pins required in an IC chip while maintaining the power utilization efficiency.